This application is a reissue application of U.S. Ser. No. 08/811,567, filed Mar. 5, 1997, now U.S. Pat. No. 5,958,309, issued on Sep. 28, 1999.
1. Field of the Invention
The present invention relates to a method for manufacturing thermo-plastic sheets having an embossed pattern, and an apparatus therefor, to be used for manufacturing sheets having a reflective capability.
2. Description of Related Art
Recently, thermoplastic sheets having a reflective capability (plastic reflection panels), which are produced by performing a microprism process on the surface of thermo-plastic sheets, have been employed in the reflection panel field, the fashion field, and the architectural field.
In the United States, reflective sheets of the glass bead type and cube corner types have been approved as sheets having a reflective capability. In general, the glass bead reflective sheets are superior for short-distance visibility, and the cube corner reflective sheets are superior for long-distance visibility and luminance because of their reflective capability.
When manufacturing a reflective sheet of the cube corner type, an embossed pattern on an embossed pattern forming die must be transferred exactly to a thermo-plastic sheet.
It is especially important when processing a thermo-plastic sheet that it be pressed against an embossed pattern die at an adequate temperature and by the application of an adequate pressure, and that the application of the pressure be continued for a predetermined period of time.
A continuous pressing method (see Japanese Examined Patent Publication No. Sho 60-56103) and a belt method (see Japanese Examined Patent Publication No. Sho 5-17023) for manufacturing reflective sheets have been proposed as conventional methods that satisfy the above requirements.
With the continuous pressing method, an overlapping solid-state sheet material is continuously fed to a belt, and a plurality of pressing means having heating and cooling functions press embossed pattern molds against the sheet material to transfer patterns to the sheet material.
With the belt method, a belt bearing an embossed pattern mold and a thermoplastic sheet are gripped by a pair of rollers and pressed together to transfer the pattern to the sheet.
The production speed attained by the continuous pressing method, however, is slow, and the apparatus that is used is large and complicated.
Since for the belt method, the belt itself carries the embossed pattern, a problem arises concerning the durability of the belt that serves as the mold.
Although not presently being used for the manufacture of reflective sheets, the sheet manufacturing techniques described in Japanese Unexamined Patent Publication No. Hei 6-55613, Japanese Unexamined Utility Model Publication No. Hei 3-6919, and Japanese Unexamined Utility Model Publication No. Hei 1-83092 could be applied for the belt method for the manufacture of reflective sheets from a sheet resin that is melted and extruded through a die by an extruder.
However, were the technique described in Japanese Unexamined Patent Publication No. Hei 6-55613 used, the sheet would be insufficiently transparent when a nucleus-generating agent did not contain any additives. Also, were the technique in Japanese Utility Model Publication No. Hei 3-6919 used, if the molding speed was increased, the surface glossiness on the belt side would be degraded because the face pressing interval would be too short. Any technique in Japanese Unexamined Utility Model Publication No. Hei 1-83092 was used, even though an elastic member was employed, as this would be a belt that was coated with rubber, a problem would arise concerning its durability.
It is therefore one object of the present invention to provide a method by which thermo-plastic resin sheets, bearing a highly transparent embossed pattern, can be manufactured with high reproducibility, and an apparatus therefor.
According to the present invention, a method for manufacturing a thermo-plastic resin sheet bearing an embossed pattern, which employs a manufacturing apparatus, includes a first roll having an elastic material coated surface, a mirror-faced metal, endless belt, which is wound around the first roll and a second roll, and a third roll so provided that the metal, endless belt is wrapped partially around the third roll, which contacts the first roll via the metal, endless belt and which has an embossed pattern formed on one surface, comprises the steps of:
feeding the thermo-plastic resin sheet, partially melted, between the metal, endless belt, which contacts the first roll, and the third roll;
elastically deforming the elastic member by application of a pressing force between the first and the third roll, while face pressure welding the thermo-plastic sheet by using the first and the third rolls to transfer the embossed pattern and to cool the thermo-plastic resin sheet;
maintaining a surface temperature for the third roll of from 0xc2x0 C. to (resin""s Tg +20)xc2x0C. when the thermo-plastic resin sheet is formed of an amorphous resin;
maintaining a surface temperature for the third roll of 0xc2x0 C. to (resin""s m.p. xe2x88x9230)xc2x0C. when the thermo-plastic resin sheet is formed of a crystalline resin; and
performing face pressure welding of the thermo-plastic resin sheet by using the metal, endless belt relative to the third roll.
The partially melted thermo-plastic resin sheet is one that is obtained immediately after the resin is melted and is extruded through the die of an extruder, or one that is obtained by heating a solid-state thermo-plastic resin sheet.
The thermo-plastic resin sheet of the present invention includes thermo-plastic resin sheets having relatively different thicknesses.
The thermo-plastic resin sheet may consist of a single layer or of multiple layers of thermo-plastic resin sheets.
The amorphous resin includes an acrylic resin, polycarbonate and poly(vinyl chloride). This resin may include a nucleus-generating agent, such as sorbitol, benzoic aluminum, polypropylene or sodium.
When the thermo-plastic resin sheet is formed of the amorphous resin and when the surface temperature of the third roll is higher than (resin""s Tg +20)xc2x0C., mold breaking tends to occur when the sheet is peeled from the third roll. Preferably, the surface temperature is (resin""s Tg)xc2x0C. or lower, and more preferably, (resin""s Tg xe2x88x9210)xc2x0C. or lower.
The crystalline resin includes polypropylene.
When the thermo-plastic resin sheet is formed of the crystalline resin, and when the surface temperature of the third roll is higher than (resin""s m.p. xe2x88x9230)xc2x0C., mold breaking tends to occur when the sheet is peeled from the third roll. Preferably, the surface temperature is (resin""s m.p. xe2x88x9250)xc2x0C., and more preferably, (resin""s m.p. xe2x88x9270)xc2x0C. or lower.
Temperature adjustment means employing water cooling, etc., can be provided for the third roll to hold its temperature within the above described temperature range.
Preferably, the temperature of the belt is maintained so that it is the same as that of the third roll.
According to the present invention, the surface roughness of the mirror finish of the metal, endless belt is, for example, 3 S or lower, preferably, 1 S or lower, and even more preferably 0.5 S or lower. When the surface roughness is greater than 3 S, it is difficult to obtain an appropriately smooth face for the thermo-plastic resin sheet.
The material used for the endless belt can be stainless steel, carbon steel, or a titanium alloy.
Although the endless belt has an arbitrary thickness, it is preferably 0.3 to 1.5 mm. When the belt is thinner than 0.3 mm, the strength of the belt is reduced and deterioration of its durability occurs. When the belt is thicker than 1.5 mm, the diameter of a roll around which the belt is installed must be increased, and the apparatus will therefore become larger. In addition, heating and cooling efficiency are degraded, and manufacturing costs are increased.
Although the kind of elastic material that is used is determined arbitrarily, a fluorocarbon rubber, silicone rubber, EPT, or EPDM can be used. The metal that is used for the elastic member may contain powdered silver or copper to provide increased thermal conductivity.
The thickness of the elastic member is preferably 1 mm or greater, and more preferably 3 mm or greater. When the elastic member is thinner than 1 mm, the elasticity effect is reduced, and the face welding pressure interval relative to the thermo-plastic resin sheet is reduced. In addition, cushioning is reduced, and a resin bank tends to occur in the portion of the thermo-plastic resin sheet fed between the endless belt and the third roll. The upper limit of the thickness is about 50 mm. When the thickness is greater than this, a problem with its durability or with its thermal conductivity tends to arise.
The embossed pattern is arbitrarily selected, but when the sheet is used as a sheet having a reflective capability, the embossed pattern is a triangular pyramidal diamond cut pattern (cube corner pattern). The size of the elements in a continuous arrangement that is employed to form the pattern is determined arbitrarily, and an element need only have a similar figure.
The first and the second rolls suffice for the rolls around which the metal, endless belt is installed. One more roll may be provided for the metal, endless belt to cool or to heat it on its way to the first roll.
According to the present invention, when a thermo-plastic resin sheet is to be fed between a metal, endless belt, which contacts the first roll, and the third roll, the thermo-plastic resin sheet contacts the endless belt and the roll at the same time. Thus, the transfer of the embossed pattern to the partially melted resin that has a low viscosity by the third roll can be accompanied by the elastic deformation of the elastic member. Then, since face pressure welding and cooling are performed by the third roll, the breaking of a mold, which occurs when the embossed pattern is peeled from the mold can be prevented. As a result, reproducibility for the embossed pattern can be increased.
When the thermo-plastic resin sheet is brought into contact with the metal, endless belt or the roll, the melting resin is cooled and solidified before the embossed pattern is transferred to it.
According to the present invention, it is preferable that the pressure during the face pressure welding of the thermo-plastic resin sheet, which accompanies the elastic deformation of the elastic member, be 0.1 MPa to 20.0 MPa, and that the pressure during the face pressure welding of the thermoplastic resin sheet, which accompanies no elastic deformation of the elastic member, be 0.1 MPa to 0.5 MPa.
When the face pressure is equal to or lower than 0.1 MPa, transfer and cooling efficiency are deteriorated. Especially when the embossed pattern is transferred to the thermo-plastic resin sheet, and when the processing speed is increased by employing a low face pressure, pattern blurring (the returning to the mold of the embossed pattern that has been transferred) tends to occur.
If the face pressure is higher than 20.0 MPa or 0.5 MPa, the belt tension is increased, and this is not preferable for its service life.
According to the present invention, an apparatus, for manufacturing a thermo-plastic resin sheet bearing an embossed pattern, comprises:
a first roll, which has an elastic material coated surface;
a metal, endless belt, which has a mirror-faced surface; and
a third roll, the face of which is an embossed pattern, the third roll being so provided as to contact the first roll via the metal, endless belt and further to wrap the metal, endless belt around the third roll.
In short, this is an apparatus for carrying out the above described manufacturing method.
Another roll may be provided along the endless belt to increase the cooling efficiency for the metal, endless belt that moves toward the first roll.
In this invention, preferably the hardness (conforming to JIS K6301 A) of the elastic material is 95 degrees or less.
When the hardness is greater than 95 degrees, the elastic property is reduced, and a resin bank tends to appear when a partially melted thermo-plastic resin sheet is brought into contact with the third roll and the metal, endless belt at the same time to perform face pressure welding. The hardness of the elastic material is preferably 60 degrees or less.